This invention relates to an improved method and apparatus for controlling the efficiency of a combustion device, particularly a furnace of the type for combusting a hydrocarbon fuel. The present invention is useful in a combustion device wherein the rate of flow of fuel, the quality of fuel or the characteristics of the combustion device itself can vary as a function of time.
Generally there are two specific types of combustion control systems: (1) direct positioning combustion control, and (2) metering combustion control. These two combustion control systems have two subsystems: (1) a fuel control subsystem and (2) a combustion air control subsystem.
An example of a direct positioning combustion control system can be found in a boiler control system wherein a pressure sensor transmits signals directly to a combustion fuel valve and to an air damper thereby controlling these inputs in order to control or maintain a fixed pressure output from the boiler associated with the combustion device. Fuel flow and air flow are not measured. Such a control system operates satisfactorily as long as the fuel has a constant BTU value. When the fuel BTU value changes, the preset relative positions of the fuel valve and air damper cannot change automatically since there is a fixed relationship between fuel flow and air flow that is calibrated into the system.
Metering combustion control systems are subdivided into three categories which control the fuel air input to the combustion chamber and the ratio of the fuel and air input. When changing the fuel air input, each of these three potential systems accomplish the change in various ways. One of the systems is known as series-metering combustion control. A second system is known as parallel-metering combustion control, and the third system is known as the lead-lag metering combustion control. All of these systems are known to those skilled in the art and are responsive to demand requirements associated with a combustion system.
The various systems also may include an automatic oxygen control or air flow correction in order to improve combustion efficiency. Typically the excess air for combustion is controlled by controlling the percent of oxygen in the flue gas. Such a system is an improvement over a manual adjustment of the fuel/air ratio because manual adjustment does not provide for automatic compensation to changes in BTU content of the fuel and therefore often requires excess air to maintain safe boiler operation. As a result, combustion efficiency decreases.
Flue gas analysis has been used as a combustion control technique for the combustion process. Typically, the flue gas includes the products of combustion, carbon dioxide and water vapor. Additionally, excess air including oxygen as well as the product of incomplete combustion, carbon monoxide, will be found in the flue gas. Finally, the flue gas may contain other gases such as nitrogen, gas compounds and solid particulates which are the product of combustion or which are impurities.
In any event, monitoring the carbon monoxide or oxygen content in the flue gas has previously been suggested as a technique for providing a control parameter for controlling the efficiency of operation of a furnace. Anson et al, in his article "Carbon Monoxide as a Combustion Control Parameter", Combustion Magazine, March 1972, discloses how carbon monoxide may be used for this purpose. Similar control techniques are discussed by Grant in "The Use of Boiler Flue Gas Analyses for Combustion Control and Oil Fired Power Plant", Oil and Gas Firing, April 1974.
Shigemura, in U.S. Pat. No. 4,162,889 entitled "Method and Apparatus for Control of Efficiency in Combustion in a Furnace" teaches a system which relies upon the theoretical oxygen level required for combustion in order to control combustion efficiency. Shigamura controls air flow to the combustion chamber by monitoring the oxygen level in the flue gas and comparing the monitored oxygen level with a set, calculated oxygen level. The set point for the oxygen may be altered from time to time by monitoring the level of carbon monoxide in the flue gas.
Various other patents teach additional types of combustion control mechanisms which seek to maximize the efficiency of the combustion process. Typically these combustion control mechanisms are used in furnace applications, for boilers and for automobile engines. Additionally, it is typical that these prior art references teach the monitoring of carbon monoxide or oxygen and then vary fuel or air input in response to the difference between the monitored values and set point values. Following is a listing of known prior art patents considered typical of the described approach:
______________________________________ Pat. No. Inventor Title Issue Date ______________________________________ 1,562,087 Griswold Method of and Apparatus 11/17/25 for Controlling Combus- tion 1,770,059 Barber Combustion Control 7/08/30 2,285,564 Brooke, Jr., Combustion Control 6/09/42 et al 2,545,732 Hamilton Combustion Control 3/20/51 3,123,295 Martin Means for Analyzing 3/03/64 Combustion Products and Varying Air Fuel Ratio Re.25,722 Dykeman, Apparatus for Con- 1/26/65 et al trolling the Operation of Multiple Combustion Zones 3,288,199 Gerrard, Low Excess Air Operation 11/29/66 et al of Multiple-Burner Residual-Fuel-Fired Furnaces 3,503,553 Schomaker Fuel Metering Combus- 3/31/70 tion Control System with Automatic Oxygen Compensation 3,514,085 Woock Combustion Chamber 5/26/70 Atmosphere Control 3,723,047 de Livois Control Network for 3/27/73 Burning Fuel Oil and Gases with Reduced Excess Air 3,745,768 Zechnall, Apparatus to Control 7/17/73 et al the Proportion of Air and Fuel in the Air- Fuel Mixture of Inter- nal Combustion Engines 3,926,154 Williams Fuel Control Systems 12/16/75 4,022,171 Laprade, Process and Device for 5/10/77 et al Controlling an Electric Valve for Regulating the Supply of the Fuel Air Mixture to Internal Combustion Engines 4,031,866 Asano Closed Loop Electronic 6/28/77 Fuel Injection Control Unit 4,032,285 Rohr, et al Method and Apparatus 6/28/77 for the Automatic Control of the Air Ratio of a Combustion Process 4,097,218 Womack Means and Method for 6/27/78 Controlling Excess Air Inflow 4,163,433 Fujishuro Air/Fuel Ratio Control 8/07/79 System for Internal Com- bustion Engine Having Compensation Means for Variation in Output Characteristic of Ex- haust Sensor 4,194,471 Baresel Internal Combustion 3/25/80 Engine Exhaust Gas Minitoring System ______________________________________
While the systems described do provide a means to increase the efficiency of combustion units and engines, the systems do not satisfactorily take into account changes in characteristics in the combustion device itself. That is, as a combustion device passes through a cycle during which it is heated, the characteristics of the device will, to some degree, change. These changes in the unit require a committent change in the fuel/air ratio in order to maximize combustion efficiency. Thus, a set point for oxygen or carbon monoxide sensing in the flue gas will not provide satisfactory control. An original base set point in the system will not provide satisfactory control. The present invention provides a method and apparatus for maximizing combustion efficiency without using fixed set points.